Conventionally, a remote control technique to lock/unlock the doors of a vehicle is widely used. A typical example is a keyless entry system. In this system, the doors are locked/unlocked if a vehicle-mounted control device receives weak radio wave that is sent from a dedicated remote controller. The dedicated remote controller sends the weak radio wave whose reachable distance is up to about 50 m to 100 m, in response to pressing of a button of the dedicated remote controller.
Further, as a developed type of the keyless entry system, a smart entry system (or passive entry system) is known. This system unlocks the doors by detecting the user approaching the vehicle, and locks the doors by detecting the user leaving the vehicle.
In these systems, a remote controller held by the user sends a predetermined ID code in response to the button operation or in response to a request signal sent from the vehicle. Then, the doors are unlocked, etc., if the vehicle-mounted control device can judge the ID code to be valid.
By using various types of entry systems as described above, the user can lock/unlock the doors without much risk that the key could be forged by the third party and without trouble of searching for the key-hole in the dark.
With the various types of conventional entry systems, however, if the remote controller, which is an equivalent of a conventional metal key, is lost or stolen, the third party who acquires or steals the remote controller may use the vehicle illegally.
Further, the user needs to carry around the remote controller for locking/unlocking the doors, in addition to a metal key to be required for starting the vehicle. Further, as vehicle-mounted devices to be controlled by remote controllers, such as an audio device and a navigation device, have been recently increased in vehicles, the handling of these controllers has become more and more complicated.